1. Field of the Invention
This invention relates to an apparatus for measuring an eye axial length wherein interference is made between light reflected by a cornea of an eye to be tested and light reflected by a retina of the eye in order to measuring the length of an eye axis.
2. Description of the Art
Heretofore, there has been an apparatus for measuring an eye axial length as shown in FIG. 6.
In FIG. 6, laser beam emitted from a semiconductor laser 1 is made into a parallel flux of light rays by a collimator lens 2. The parallel flux of light rays are guided to an eye E through an amount of light adjusting filter F and a beam splitter 3. The laser beam is projected onto the retina Er of the eye so as to be converged upon the retina Er by the crystalline lens, e.g., of the eye. And light reflected by the retina Er and light reflected by the cornea Ec, which both have passed through a lens 4, are interfered with each other on a pinhole plate 6. Light interfered thereon is then received by a light receiving element 5 to vary the wavelength of the laser beam. Light receiving signals outputted from the light receiving element 5 are processed by a signal processing circuit 7 and the length of an eye axis (a visual line) between the retina Er and the cornea Ec is measured.
However, in the conventional apparatus for measuring an eye axial length, the light reflected by the cornea Ec becomes scattered light since the parallel flux of light rays are projected onto the cornea Ec. On the other hand, the laser beam is projected onto the retina Er so as to be converged upon the retina Er, as mentioned above, and the reflected light by the retina Er is transformed into a parallel flux of light rays by the refracting function of the crystalline lens or the cornea Ec. Therefore, a difference in wave front between the reflected lights by the retina Er and the cornea Ec is distinctive because one is scattered light and the other one parallel light. That is, there are the drawbacks that a space between the bands of interference fringes is narrow, only a light receiving element with a small area for receiving light must be used, and light receiving signals are weak. In the case of a light receiving element with a large area, a plurality of interference fringes are simultaneously received and light receiving signals corresponding to the interference fringes are averaged in varying wavelengths although the amount of light of the light to be received is increased. This means a similar phenomenon to no fringe after all. Hence, it is impossible to obtain light receiving signals corresponding to interference fringes. Further, owing to a coarse surface of the retina Er, the intensity of interference fringes is so weak that a clear distinction between the light receiving signals of interference fringes obtained by the light receiving element and noises is not easily made.